The inflammatory bowel diseases (IBD) affect over a million people in North America. Many patients with IBD do not respond initially or lose response to even our most effective therapies (anti-TNF antibodies). Thus, there is an unmet need for novel therapeutics. Blocking leukocyte traffic is a proven therapeutic option in IBD. This pathway might be targeted by monoclonal antibodies, such as natalizumab/vedolizumab or alternatively by orally administered small molecules, such as sphingosine-1-phosphate (S1P) receptor. Indeed, a drug that interacts specifically with S1P receptor-1 (S1P1) (i.e. RPC1063) has entered the clinical trial stage in IBD, while another is already FDA-approved for multiple sclerosis (i.e. FTY720). However, a critical need remains to understand the mechanism/s through which these drugs modulate chronic inflammatory diseases. Our long- term goal is to understand how to manipulate the S1P pathway for therapeutic purposes in IBD. Our immediate objective is to understand the mechanism of action of novel S1P1-selective agonists with proven efficacy in preclinical therapeutic studies. Based on the predominant expression of S1P1 on lymphocytes and endothelial cells, we hypothesize that S1P1-selective agonists act both on leukocytes, (by promoting T cell egress, Aim 1) and/or on endothelial cells (by modulating endothelial function, Aim 2). This hypothesis stems from our preliminary data that demonstrates that chronic inflammation alters S1P synthesis and degradation (promoting T cell retention) in IBD mouse models and human IBD and that S1P1-selective agonism degrades S1P1, increases recirculation of effector T cells and ameliorates inflammation in clinically-relevant IBD mouse models. Our innovative approach takes advantage of 1. Chronic mouse models of IBD that recapitulate many of the characteristics of the human disease, 2. New drugs with distinct downstream actions and 3. Cutting edge microscopic techniques that allow us to directly visualize the effects of these drugs on a living animal. Our inter-institutional studies bring together our extensive expertise in lymphocyte traffic/IBD models at UCSD with that of The Scripps Research Institute: S1P pharmacology/endothelial cell biology, which allows us to consolidate valuable infrastructural and mouse model resources. Our rationale is that understanding the mechanism of action of these novel anti-inflammatories will lead to optimized drug design and minimize the risks related to the pleiotropic effects of non-selective S1P receptor agonists (e.g. FTY720) on cellular processes. The proposed research is significant as this pathway is evolutionarily conserved in mice and humans. Therefore, our results might directly translate to human IBD, enabling us to elucidate the mechanism of action of S1P1-selective agents. Furthermore, we may uncover new targets for therapeutic intervention within the S1P pathway (kinases, lyase) to be modulated pharmacologically with oral drugs, at reduced cost of production and administration, compared with current antibody-based biologic strategies.